Computer software is typically designed for use in a uniprocessor environment wherein a single processor communicates with one or more input/output (I/O) devices on a single I/O bus. Programmed input/output (PIO) instructions wherein the software communicates with an I/O device are all performed on the same I/O bus.
Multiprocessor systems having two or more concurrently operating processors are utilized to increase processing capability. A single I/O bus may be connected to one of the processors. All I/O must be performed by the associated processor, thereby placing a heavier workload on the processor to which the I/O bus is connected. The I/O capability of the multiprocessor system is limited by the capabilities of the single I/O bus.
Multiple I/O buses can be connected to the multiple processors to increase I/O speed and capacity using existing uniprocessor and uni-I/O bus communication design techniques. Each processor can only communicate with its associated or "local" I/O bus. However, a serious software compatibility problem arises. The software running on the multiprocessor system may attempt to perform an I/O process containing PIO instructions referencing I/O devices located on any one of the I/O buses in the system. As used herein, an I/O process is any process that attempts to execute a programmed I/O instruction, and a process, or task, is a single program entity in a multiprogrammed system. When the software running on a particular processor references an I/O device connected on the local I/O bus, no problem occurs. However, when the software references an I/O device connected on a remote I/O bus, the software (originally designed for a uniprocessor system) does not know that the device is connected to a remote I/O bus and does not know if there is a communication link to the referenced I/O device.
One solution is to provide a hardware connection between each processor and each I/O bus. However, the system design is unduly complicated because additional hardware is required for the bus interconnections, and control hardware is required to arbitrate processor requests for use of each I/O bus. When each I/O bus is not physically interconnected with each processor, the PIO instruction can only be performed on the processor associated with the I/O bus where the referenced I/O device is connected. However, it is desirable to allow software to reference the I/O devices on more than one I/O bus. Also, it must be guaranteed that PIO is only attempted on the processor associated with the I/O bus containing the referenced device.
It is a general object of the present invention to provide a method for performing programmed input/output instructions in a multiprocessor system having at least two I/O buses, without unduly complicating the hardware.
It is another object of the present invention to provide a method for performing programmed input/output instructions so as to provide software compatibility between a uniprocessor system and a multiprocessor system having at least two I/O buses.
It is a further object of the present invention to provide a method for performing an input/output process containing a programmed input/output instruction in a multiprocessor system having at least two I/O buses by forcing the input/output process to run on the processor associated with the I/O bus to which a referenced I/O device is connected.